Traditionally the halftoning for images for printing is done using only two levels. The continuous tone density value of a pixel to be reproduced is approximated by printing an appropriate percentage of high density dots within the area representing the pixel. At a particular position either a dot or no dot can be placed.
Today, ink-jet as well as electrophotographic printers exist that render N>2 intensity or density levels. The inkjet printers are able to deliver variable droplet sizes or use multiple inks of the same hue, but different densities, both procedures resulting effectively in reproduction of multiple possible density levels for one printed dot. Halftoning algorithms, such as error diffusion may be extended to the multilevel (i.e. N>2) case. See e.g. U.S. Pat. No. 4,680,645 by Dispoto et al describing a method for rendering a grey scale image with variable dot sizes. A continuous tone image is an image containing multiple grey levels with no perceptible quantization to them. In a different multilevel halftoning technique all different continuous tone pixel values within a range (e.g. 0–255) are mapped onto the N allowable values within the range N<256. These N allowable values correspond to the N density levels which can be rendered by the multilevel system.
The basic error-diffusion algorithm is illustrated in FIG. 1 and works as follows. The first pixel value 21 of the original image 22 is quantized by a quantizer 23 to the nearest allowed value to obtain the output pixel value 24. The quantization error 25, i.e. the difference between the continuous tone input value 21 and the output pixel value 24, is diffused to future pixels. This may be done by storing the error 25 into a special buffer 26. For the next pixels the method quantizes, the modified pixel value 27=(original pixel value 21+error 25 received from past pixels which is read from the buffer 26) to an allowed quantization level to obtain an output pixel value 24 and again diffuses the newly obtained error 25 to future pixels via the error buffer 26. A more elaborate description can be found hereinafter. Several refinements can be built into the algorithm to avoid the creation of artefacts, such as worms. Examples of such refinements can be found in “Digital Halftoning”, R. Ulichney, Cambridge Mass., MIT Press, 1987.
Using more than two allowed values (density levels) i.e. “on” and “off” or high and low density enhances the image quality of the halftone image 28 a great deal compared to only using minimum and maximum values (density levels).
Due to the diffusion of the error 25 however, the output pixel value 24 (density level) wanders around over several allowed output levels of the whole range in a multilevel system. This wandering is fortified when additional features are built into the algorithm, such as putting noise on the diffusion weights (Ulichney) or on the quantizer 23, or using an imprint to get a more homogeneous point distribution, e.g. U.S Pat. No. 5,535,019 Eschbach. The imprint provides a homogenous printed spot distribution in highlighted and dark areas by an increase or decrease of the threshold level based upon a regional input level. When a white pixel is set the threshold is raised while the threshold is lowered when a black pixel is set providing less chance obtaining that another white or black pixel respectively is printed.
This causes that sometimes output pixel values 24 are further away from the original input pixel values 21 than is strictly necessary or desirable. This results in the production of intensity levels giving unwanted, noticeably contrast between halftone dots giving rise to a noticeably more grainy image. The described phenomenon happens mostly at tone levels close to a quantization level. A solution to this problem has not yet been provided in the prior art methods.